All About Foams for Walkalong Gliders
By Slater Harrison, AKA sciencetoymaker

What is EPS foam?

Not only is the sheet of printer paper 8 times heavier than the sheet of foam; it's still not stiff enough to keep from bending too much. Everybody has wishful-thinking that paper will work well for walkalong gliders. It doesn't. And thin 1/2 millimeter foam is inexpensive to buy, or you can even slice your own.


Why is EPS foam better than paper for walkalong gliders?
What is Time Warp North America foam?
What is Time Warp Asia foam?
I see pinholes in my sheets of foam—is that a problem?
Can I slice my own foam?
Paper vs. EPS foam: The numbers

Measuring foam density

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What is EPS foam?

EPS (expanded polystyrene) foam is made of spheres (beads) that appear as circles when cut thin. The holes between the circles rattle some people, but do not affect flying.Foam gliders are superior, both for beginning and advanced flight.

Expanded PolyStyrene (EPS) foam is ubiquitous. It’s the white foam with circles, used to make everything from foam cups to foam coolers to molded electronics packaging to sheets of rigid insulation. It is made from hard, dense plastic polystyrene beads that are “puffed up” (expanded) like popcorn or puffed rice: mostly air. The expanded spheres are fused together.

Expanded Polystyrene foam is made in different densities (the ratio of weight to volume). In North America, density is usually still measured in pounds per cubic foot. In most of the rest of the world it is measured in kilograms per cubic meter.

There are lots of other kinds of foams—and I have experimented with many—but EPS always comes out on top, because it has the highest strength/weight ratio.

Why is EPS foam better than paper for walkalong gliders?

How EPS Foam Stacks up Against Paper for Gliders It is difficult for people to understand that paper is very heavy when compared to foam. But imagine how heavy these stacks would be if they were books or newspapers instead of foam! The foam is cut into thin (1/2 mm) sheets, many times lighter than a sheet of paper of equal rigidity.

Everybody wants to use paper because we are so familiar with paper airplanes and origami. But to match the rigidity of a .6mm slice of EPS foam requires a thickness of paper that is many times heavier. Weight is bad for gliders. There is almost no comparison and that’s not even considering that in humid weather, paper becomes completely limp and useless.

Foam used for disposable plates and trays is too thick and heavy for walkalong flight. The EPS foam needs to be sliced to .5 to .6 millimeters thick. This is usually done with ni-chrome “hot-wire” cutting.

What is Time Warp North America foam?

The best all-around foam is called Time Warp North America (TWNA) foam. It is relatively strong, lightweight and inexpensive (as low as 10 cents per sheet in large purchases). Time Warp N. A. is the lowest-density EPS foam made in North America. Its density is 11.2 kilograms per cubic meter (.7 pound per cubic foot). Although it is heavier than Time Warp Asia foam (see below), TWNA foam is more rigid, so it can be sliced a little bit thinner. Furthermore, the North American foam is a little more resistant to tearing and overall slightly more durable.

We are indebted to some kind people at an EPS foam company in Ohio--Foam Packaging Materials--for suppying the lowest density foam in North America. The President Jeff Gross donates the foam in support of science education. Jeff travels a lot and has even detoured his minvan to drop off end-slabs of foam to me in Pennsylvania. When I talk to Jeff about technical details, it's obvious how much he cares about detail and quality. I've worked with lots EPS foam, and the EPS from Foam Packaging Materials is consistantly the best.

Unless otherwise specified, the gliders and foam sheets sold by sciencetoymaker.org are Time Warp North America. Time Warp Asia foam (see next entry) is sold as a specialty foam.

What is Time Warp Asia foam?

There is also a special foam called Time Warp Asia (TWA). If I understand correctly, in Asia (particularly China; I've heard also in India) the manufacturers are allowed to use butane as an expanding agent. Most industrialized countries use pentane instead; butane is considered too explosive during manufacture (the butane is removed after expanding—the danger is only during manufacture). Time Warp Asia foam can have density of 4.5 kg/m3. If you consider that air itself has a density of about 1.25 kg/m3 (depending on altitude, temperature, etc.) then TWA foam is only about 3 or 4 times heavier than air, which is quite remarkable! By comparison, water is about 800 times more dense than air and paper is over 500 kg/m3 density. Time Warp Asia foam is slightly less rigid than Time Warp U.S. foam and also a little more delicate to handle (rips more easily). But if you can handle it gently, the gliders made from it fly so slowly it's like magic!

My good friend Rob Beiter manages several furniture/appliance stores in central Pennsylvania and has better things to do than collect foam scraps from packaging. And yet he kindly directs the rare bits of TWA foam from imported furniture packing my way because he supports recycling, science education and non-profit organizations like the Physics Factory.

Although Rob gives us the TWA foam free, there is still a lot of processing. TWA foam is not actually one foam, but many similar foams from 4.5 to 7 kg/m3, manufactured throughout China. Each kind has it’s own personality and cuts differently. So recycled TWA takes lots of time for me to sort through and collect (and there's not much of it), so it’s therefore more expensive. You might be able to find and recycle it locally—I recommend that highly. I send out a sample of TWA foam with all foam-cutting kits, so people can become familiar with it.

Under ideal circumstances (dead still air, people who can handle it very carefully) the ultra low-density Time Warp Asia foam has an almost magically slow glide! That slow motion gives beginners lots of time to think and react, so I like to use it when teaching people how to fly. TWA gliders are also superior when flying in small rooms and hands-only flight.

I see pinholes in my sheets of foam—is that a problem?

Because EPS foam is made of expanded spheres, fused together, there are tiny voids between the beads. When we slice the foam very thin, the voids appear as tiny holes. I have experimented with solid foams like Depron that do not have voids, but invariably they are less satisfactory because they weigh much more.

The pressure differential between the top of the gliders and the bottom is very small. The relative viscosity of the air going through tiny holes is high. So the pinholes might be an esthetic problem, but not a flight problem. The lowest-densityAsian foam (4.5 KG per Kilogram) has bigger beads, and therefore bigger voids (again, they do not negatively affect flight but do rip easier).

Can you slice your own foam?

The hot-wire kit provides the difficult-to-find parts, while you supply the bulky things that you either have or can buy inexpensively, locally.

Yes, you can collect and slice your own foam. This page and associated video show how. Flying even with heavy foam is many times better than flying with the lightest paper gliders. And if you can find packaging foam from China (see Time Warp Asia foam section) you will love it. Cutting foam is quite easy when you have Nickel-Chromium (ni-chrome) wire, which is special for two reasons. It is high resistance, so it creates heat instead of a short circuit; and then it stays strong even when hot (unlike regular steel wire). Almost all electric heating devices--from hot plates to water heaters, soldering irons to coffee makers--use ni-chrome wire. It's the stuff that glows orange in toasters, although we don't use it that hot to cut foam.

Sciencetoymaker.org is selling a kit that includes all the things that might be difficult to find locally: two kinds of nickel-chromium resistance wire for hot-wire cutting; special machined bronze pieces for getting precision thickness, a sample of very low-density foam, etc. and some alligator clips. Then you provide things that are easily found locally—often free: a flat board, a car battery charger (somebody you know has one in their garage), a threaded rod (very inexpensive, from any hardware store) and some basic tools like a drill.

Hot wire cutting produces some smoke. You are responsible for cutting in a well ventilated place, such as a garage with the big door open; or under a stove exhaust hood.

Paper vs. EPS foam: The numbers

10 sheets of paper weigh over 16 grams.

The density of EPS foam is as low as 6 kg/cubic meter (Asian foam). Time Warp North America foam is 11 kg/m3 (and stronger). Paper is 540 kg/m3. That's not really fair because paper is thinner than foam sheets used for gliding.

So lets set up a fair comparison. 10 sheets of EPS foam, cut to the standard size that I use for both Mosquitos and big Mama Bugs (219 mm by 96 mm) weigh 1.3 grams (Time Warp Asia) or 2 grams (Time Warp North America). 10 sheets of printer paper cut to exactly the same size weigh 16.6 grams. That's more than 8 time heavier than even the heavier foam.! Furthermore, the foam is much stronger. Holding the end of a piece of foam so that it sticks out horizontally like a diving board, foam bends just a little bit. But paper cannot hold itself rigid at all. You can say that all you have to do is fold/bend the paper so as to increase its rigidity, and many people have done just that: developing clever designs to maximize the strength of paper with folds. But in the end, paper walkalong gliders are far heavier, much more difficult to trim (adjust) and they fly so fast that it takes hours for newbies to be able to fly. And that doesn't take into account the fact that paper becomes even more limp and useless in all but the lowest humidity air.

10 sheets of thin-cut EPS foam weigh only a couple of grams, and the foam is still more rigid than the paper

I started with paper gliders because I did not know about thin foam gliders. It was frustrating to try to teach my students how to fly walkalong gliders. It took a long time and many students never got it. It only worked in winter when the air was very dry. When I started using foam, I could get everybody flying. Some kids still worked harder and excelled more than other students, but everybody got into the air. I have not worked with paper gliders for years, and I don't miss it.

 

Measuring foam density.

Density is weight divided by the volume (length x width x heght) of the foam block. In North America it is expressed as pounds and feet, but I (and the rest of the world) prefer metric kilograms per cubic meter. You can easily convert to pounds/foot if you need to.

I use a triple beam balance to measure weight but you could use a postage scale. I express 6 grams as .006 of a kilogram in my equasions; 22 cm as .22 of a meter; 95 mm as .095 or a meter, etc.

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